(E) For a problem where a job size remains the same, but the number of workers changes, there is a very intuitive way to get to the answer, without using algebra. For example, if we double the number of workers on a job, it should take half the time. If we triple the number of workers on a job, it should take one-third the time. Similarly, if we use one-half the number of workers on a job, it should take twice the time. There is a nice reciprocal relationship here, so even when the numbers are not as easy to deal with, the relationship still holds.

So, in increasing the number of plows from 5 to 6, we are using 6/5 as many plows. So clearing the lot should take 5/6 the time:(5/6)(12) = 10 = 10 hours.The correct answer is choice (E).-------------

For 5 snow plows to take 12 hours, would each take 5/12 or 2.4 hours and then you combine them?

1) The common approach for this type of the questions is to calculate the rate of a unit. In other words "how much of the parking lot does a snow plow clear in 1 hour"?We start with the fact that 5 snow plows clear the parking lot in 12 hours. On the diagram we show a rectangular parking lot, but the shape of the parking does NOT affect the reasoning.

2) Since 5 snow plows clear the iced parking lot in 12 hours, then each of the snow plows clears 1/5 of the parking lot.

3) Since a snow plow clears 1/5 of the parking lot in 12 hours, then in 1 hour it clears (1/5)/12 of the parking lot. (1/5)/12 = 1/(5 × 12) = 1/60Now we have a rate for one snow plow. That's the key moment to all such questions.

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